1. Field of the Invention
The present invention relates to a method of recovering electrochemical properties of a cation exchange membrane of fluorinated polymer used for an electrolysis of an alkali metal chloride. More particularly, it relates to a method of recovering electrochemical properties of a cation exchange membrane of fluorinated polymer which has deteriorated electrochemical properties caused by producing an alkali metal hydroxide and chlorine gas by an electrolysis of an aqueous solution of an alkali metal chloride.
2. Description of the Prior Art
It has been known to produce chlorine in an anode compartment and alkali metal hydroxide in a cathode compartment by partitioning the anode compartment and the cathode compartment with a diaphragm and feeding an aqueous solution of an alkali metal chloride into the anode compartment to electrolyze it. The method has been known as a two-compartment type diaphragm electrolysis.
Heretofore, an asbestos diaphragm has been usually used in the method. When the asbestos diaphragm is used, the resulting alkali metal hydroxide is contaminated with an alkali metal chloride to lower the purity and the asbestos has low durability.
Thus, it has been proposed to use a cation exchange membrane of fluorinated polymer which does not substantially pass the electrolyte and selectively pass only alkali metal ions and has high alkali resistance and chlorine resistance.
It has been found that the cation exchange membranes of the carboxylic acid type fluorinated polymer are superior to cation exchange membranes of the sulfonic acid type fluorinated polymer as the diaphragm for the electrolysis of an aqueous solution of an alkali metal chloride because the alkali metal hydroxide can be produced in high current efficiency even though the concentration is high and the electric resistance of the membrane is not increased.
Thus it has been found that higher than 90% of the current efficiency can be given under the cell voltage of 2.5 to 4.0 volts, even though the concentration of the alkali metal hydroxide is high to be 35 to 45wt. %, when an aqueous solution of an alkali metal chloride is electrolyzed by using the cation exchange membrane of the carboxylic acid type fluorinated polymer as the diaphragm.
However, when the electrolysis is continued for a long time, the electrochemical properties such as the current efficiency and the cell voltage of the cation exchange membrane of the carboxylic acid type fluorinated polymer are gradually deteriorated. The reason is not clear, however, it has been considered that the deterioration of the electrochemical properties has been caused by a change of mechanical property and a precipitation of sparingly soluble calcium and magnesium hydroxides on or in the membrane under the condition of the electrolysis.
The deterioration of the electrochemical properties of the membrane substantially prevent an industrial use of the membrane in the electrolysis. The cost of the membrane is relatively high. Accordingly, it has been important to recover the electrochemical properties of the membrane by reproducing the cation exchange membrane.